I found a checkbox for displaying area of selected polygons, and I checked it, but I don’t know where it is displayed. It says, in the hover popup that the area is shown in “the transform panel”, but I have no idea where that panel is. T.I.A.
It’s drawn in the 3d viewport window above each face individually. If you want to see the sum of the area of the selected faces, enable measureit addon in user preferences.
Many thanks!